Two types of clinical tools for monitoring cardiac operation are systems for recording electrical behavior and systems for imaging mechanical behavior. With regard to electrical behavior, cardiac depolarization occurs in a wave to induce a contraction sequence. As depolarization spreads throughout the muscle cells of the heart, electrical currents spread to surrounding tissues, creating potential differences on the body surface. An electrocardiogram (ECG) utilizes a number of leads to measure and record these potentials. ECG data is used for the diagnosis of some pathological processes. However, the ECG is limited in the detail of information it can provide.
One area of research into imaging the cardiac motion is radionuclide ventriculography. Radionuclide ventriculography involves injection of a radioactive tracer into the bloodstream. The radioactive tracer emits radiation in the form of a gamma ray or an x-ray when spontaneously decaying to a more stable form. Dynamic cardiac images can then be generated by displaying time-accumulated radioactivity from a gamma camera on a pixel-by-pixel basis. Cardiac contraction sequence information is obtained using phase imaging. In phase imaging, the time-accumulated radioactivity of each data point in the planar blood-pool image is the scalar plotted over a cardiac cycle. A functional image is then generated from the phase of the first harmonic of the time activity curve at each pixel value. One drawback of radionuclide ventriculography is that contraction sequence information is limited to the endocardial border of the heart, which is the only region of the heart in contact with the blood pool.
Another area of research into imaging cardiac mechanical motion is high-speed digital subtraction echocardiography, which has been tried specifically for the direct visualization of the focus of ventricular premature contraction. This research is described in Ishihara et al., "Direct Visualization of the Forces of Ventricular Premature Contraction by High-Speed Digital Subtraction Echocardiograph," Abstract of Paper to be considered for presentation at American College of Cardiology, 40th Annual Scientific Session. B-mode echocardiograms were obtained at a rate of 164 frames per second. This allowed the crude visualization of myocardial contraction, with the wave of contraction progressing approximately 1/3 down the septum between each frame. Sectors of 30.degree. limit visualization only to the septum. Visualization of cardiac contraction in other portions of the heart, such as the apex or the free ventricular wall, have either not been attempted or are not possible given the existing technology. Similar approaches for delineating the contraction sequence involve the subtraction of paired angiography data frames. Digital subtraction angiography has been attempted for the assessment of the ventricular contraction sequence and location of accessory pathway sites in patients with Wolff-Parkinson-White syndrome.
The idea of phase imaging has also been applied to echocardiography, as described in Kuecherer et al., "Two-Dimensional Echocardiographic Phase Analysis," Circulation, Vol. 85, No. 1, pages 130-142, January 1992. Data obtained from transthoracic echocardiograms was recorded on a video tape. The study included acquiring a 16-frame digital cine loop from the video tape and performing a phase analysis based upon pixel intensity of the video data. The phase analysis yielded images which allowed localization of accessory pathways with a high reliability, but details of cardiac operation were not obtainable. Therefore, further improvements to the study of cardiac performance are desired.
It is an object of the present invention to provide a method and system for improved delineation of the sequence of mechanical motion of a heart.